(1) Field of the Invention
The invention relates to the general field of electroluminescent phosphors, more particularly to methods for metallizing them.
(2) Description of the Prior Art
It is common practice to overcoat phosphor layers, such as those used in cathode ray tubes and similar devices, with a thin layer of a light metal, such as aluminum. Such a layer is both thin enough and light enough so that electrons, coming from the cathode, will pass through it and excite the phosphor to luminescence. Among the functions that such an overcoat may serve we include: (1) acting as an anode for the display (2) neutralizing surface charge that would otherwise build up in the phosphor (3) preventing phosphor contamination (4) improving phosphor efficiency by outward reflection of inwardly directed luminescent light (5) increasing the adhesion of the phosphor to the substrate and (6) preventing or slowing phosphor aging.
There are a number of methods already on record in the prior art for metallizing phosphor layers but few, if any, of them delivers all the above benefits as part of a single process. In particular, it is important that the metal film be deposited onto a surface that is as planar as possible. If this surface is rough, function (4) above cannot be effectively realized and, since the metal film is typically much thinner than the average phosphor particle size (less than 2,000 Angstroms as compared to 4-5 microns), it will lack integrity and contain many pin holes so that functions (3), (5), and (6) above will not be realized either.
The following patents describe methods for metallizing phosphor layers. These methods can be seen to be substantially different from those taught by the present invention.
Patel (U.S. Pat. No. 5,145,511 September 1992) shows a method for metallizing a phosphor layer wherein a novel aqueous filming emulsion with low organic content is used. This film is deposited over a phosphor particle layer prior to the deposition of an aluminum layer.
Utsumi (U.S. Pat. No. 5,418,075 May 1995) first creates a thermal transfer foil by coating a film of polyethylene terephthalate with a stripping layer, an aluminum layer, and a phosphor layer. This is pressed against a glass substrate and heated, thereby transferring the phosphor and the aluminum to the substrate.
Jung (U.S. Pat. No. 5,344,353 September 1994) coats a deposited phosphor layer with a lacquer-like material following which it is subjected to a hardening process. The lacquer is overcoated with a thin layer of aluminum after which the lacquer is left in place. In our experience, lacquers represent a possible source of outgassing and should not be left behind in the completed cathode ray tube.
Thomas et al. (U.S. Pat. No. 5,360,630 November 1994) describes the formation of a phosphor screen at one end of a fiber-optic bundle. The cladding layer is removed from the fiber ends and is replaced with phosphor onto which an aluminum layer is then deposited.